The use of single-mode optical fibres in fibre-to-the-premises (FTTP) applications, including fibre-to-the-home (FTTH) and fibre-to-the-building (FTTB) applications, generally require low bending loss of optical signals transmitted through the fibres, also under stringent installation constraints that may impose tight bend radii, e.g., due to sharp cornering in buildings or compression of optical fibres. In particular, cabling and hardware applications aimed to miniaturize passive field equipment, e.g., local convergence cabinets or storage boxes, and the development of multi-dwelling units (MDUs) require fibre designs with superior bending capabilities. In addition, coarse wavelength division multiplexing systems (CWDM) and passive optical network (PON) systems may also need employment of bend-insensitive optical fibres.
In order to standardize the bending performance of optical fibres particularly suited for FTTP applications, the ITU-T (International. Telecommunications Union, ITU Telecommunication Sector) has recently developed recommendation G.657 (11/2009) that defines single-mode fibres with enhanced bending performance.
To conform to international standards, besides bending capabilities, fibre performance can be evaluated also on other relevant optical parameters such as the cut-off wavelength and the mode field diameter (MFD). A parameter that can be useful for finding a compromise among the MFD, the cut-off wavelength and bending losses is the so-called MAC number, which is the ratio of the MFD and the cut-off wavelength.
It has been observed that in order to obtain low bending losses, the MAC number should be reduced. In Characterization of the bending sensitivity of fibres by the MAC value, by C. Unger and W. Stocklein, published in Optics Communications, vol. 107 (1994), pages 361-364, macro- and micro-bending performance of matched-cladding fibres and correlation with the MAC number are investigated. The articles states that the bending behaviour of step-index fibres is completely characterized by the MAC number and that macro- and micro-bending losses increase with increasing of MAC number.
Patent application WO 2009/154712 describes an optical fibre cable comprising two optical fibres, each fibre being a microstructured bend performance fibre. Optical fibre cables of the disclosed solution are said to allow aggressive bending such as for installation, slack storage, and the like while inhibiting bend radii that allow damage and/or breaks of the optical fibre.
During installation and deployment of optical cables, in-field testing of optical attenuation is generally performed in order to detect faults in fibre networks. Such optical tests are performed by means of transmission and/or reflection measurements, and typically by optical time-domain reflectometry (OTDR) techniques.
U.S. Pat. No. 7,167,237 discloses a fault searching system for an optical line from a user optical terminal to an optical divider, without affecting other user optical terminals and transmission apparatuses, from the user optical terminal side by connecting an OTDR being to a terminal portion of the optical line in the user optical terminal, and by using at least one of those in which a value of a wavelength used in the OTDR connected is set to a value different from a value of a wavelength used for data transmission in the optical line, and a peak level of an optical pulse is set to a predetermined level or less. The wavelength of the test light is a wavelength different from the wavelength of the light used for data transmission in the optical line, in particular a value included in one of ranges being greater than or equal to 0.3 μm and less than 1.3 μm or greater than 1.65 μm and less than or equal to 2.0 μm.
K. Clarke and A. Duncan in “Development and Application of a Long wavelength OTDR to provide Early warning of Degradation in a Fibre Network”, published in the Proceedings of the 18th Australian Conference on Optical Fibre Technology, 28 Nov.-1 Dec. 1993, pp. 31-34, describes an OTDR operating at 1662 nm for out-of-band monitoring to overcome the disadvantages of monitoring at wavelengths that are the same as those used to carry traffic on the fibre.
D. Miles in “Locate fiber optic cable interrupting service”, published in Test, vol. 28, no. 2 (March 2002), pp. 8-10, describes tests with a multiple-wavelength OTDRs that test out-of-band wavelengths. Higher wavelengths, i.e. 1625 nm, are said to be highly effective for spotting bends that can later develop into breaks or stress- or temperature-losses.
Patent Abstracts of Japan of JP 4-351935 describes a light pulse testers which use longer and shorter testing wavelength than the signal wavelength and a light band path filter. The abstract reports that the backscattering obtained with the longer wavelength testing light is to sensitively test and monitor the bending loss of the optical fibre, whereas the backscattering obtained with the shorter wavelength testing light is to sensitively test and monitor the connection loss of the fibre.
Patent application JP 2-027231 relates to an optical fibre lengthwise distortion measurement method using a first-wavelength light formed from the wavelength used in optical signal transmission in the optical fibre being measured as well as a second-wavelength light formed from at least one of the wavelengths with a bigger occurrence of optical loss for the respective types of distortion amounts in the optical fibre being measured than the wavelength of the first-wavelength light, the entry of each optical pulse of the first-wavelength light and second-wavelength light from one end of the optical fibre being measured, and the measurement of distortion such as bending of the optical fibre being measured based on comparative evaluation of the difference of the attenuation amount of each optical pulse of the first-wavelength light and second-wavelength light reflected in the optical fibre being measured as well as the difference of the variation over time of the attenuation amount.
OTDR tests can provide useful information about the possible presence of tight bends, in particular bends of a radius smaller than the minimum radius recommended by the manufacturer, which might adversely affect the mechanical lifetime of the fibre.
Tight bends may be introduced accidentally during laying down or be the result of less careful deployment practices that may be used to speed up the installation process, such as aggressive stapling of optical drop cables and sharp-angle deployment around corners. In particular, a tight bending radius may cause cracks on the surface of the optical fibre, which gradually extend and may eventually lead to a permanent damage or rupture of the fibre. Long-time mechanical reliability of the fibre can depend on the capability of detecting the presence of dangerous bends.
Patent application US 2006/0115224 describes a single-mode optical fibre having a cut-off wavelength in a 1.31 μm wavelength band, in which a relative refractive index difference of the core with respect to the cladding is adjusted such that a bending loss, when a bend is applied in a radius smaller than a limit bending radius, becomes greater than a detection limit value, the limit bending radius being calculated from a relationship between a bending radius applied to the optical fibre and a failure probability occurs after a predetermined time period. In an embodiment, the fibre has a step-index profile. Disclosed results from a step-index optical fibre indicate that, when the refractive index difference between the core and the cladding is greater than about 0.80%, the loss caused by a bend is reduced to about 0.01 dB/turn or less in a limit bending radius of 5.5 mm, which it is said to make measurements using an OTDR difficult. Optical measurements on the fibre are performed at transmission wavelengths of 1.31, 1.55 and 1.625 μm.
Patent application EP 1 256 826 A2 discloses an optical fibre design constructed to prevent theft of optical signals. One technique to allow tapping of a portion of the signal theft of optical signals is to form a bend in the fibre that causes “leaking” of the optical signal into the fibre cladding where it can be intercepted without the source or the receiving station. This case of intrusion is addressed by increasing the sensitivity of the optical fibre to microbending loss to the extent that bends in the fibre causes such a high attenuation that bends do not go undetected. A high bend sensitive fibre is produced by introducing an undoped outer ring region at a substantial distance from the fibre core. The document mentions that such bend sensitive designs do not adversely impact transmission properties, provided that the fibre cable, is installed to have a large minimum bending radius.
Patent application PCT/EP2008/065174 filed by the instant Applicant on Nov. 7, 2008 relates to an optical cable having a cable length extending from an input end to an output end and comprising at least one single-mode optical fiber having a cable cut-off wavelength of from 1290 nm to 1650 nm, wherein said at least one optical fiber is helically twisted around a longitudinal direction for a twisted length L with a twisting pitch P, the values of L and P being selected such that the optical cable exhibit substantial single-mode transmission, and wherein said twisted length L extends along at least a portion of said cable length. In particular, the twisted length L and the twisted pitch P are selected in such a way that the measured cut-off wavelength in the optical cable is equal to or lower than 1260 nm.